Hydraulic fracturing or “fracking” is the propagation of fractures in a rock layer by a pressurized fluid. The oil and gas industry uses hydraulic fracturing to enhance subsurface fracture systems to allow oil or natural gas to move more freely from the rock pores to production wells that bring the oil or gas to the surface. However, there are many uses for hydraulic fracturing outside of the oil and gas industry, including to stimulate groundwater wells, to precondition rock for cave in mining, to enhance waste remediation processes, to dispose of waste by injection into deep rock formations, including CO2 sequestration, to measure the stress in the earth, and for heat extraction in geothermal systems.
The two main purposes of fracturing fluid or “frack fluid” in oil formations is to extend fractures in the formation and to carry proppants, such as grains of sand, into the formation, the purpose of which is to hold the fractures open without damaging the formation or production of the well. Two methods of transporting the proppant in the fluid are used—high-rate and high-viscosity. High-viscosity fracturing tends to cause large dominant fractures, while with high-rate (slickwater) fracturing causes small spread-out micro-fractures.
A variety of chemicals can be used to increase the viscosity of the frack fluid in high viscosity fracturing. However, common and inexpensive thickening agents include plant-based polysaccharides, such as cellulose, guar gum, xantham gum, and their derivatives. In fact, more than 65% of conventional fracturing fluids are made of guar gum (galactomannans) or guar gum derivatives such as hydroxypropyl guar (HPG), carboxymethyl guar (CMG) or carboxymethylhydroxypropyl guar (CMHPG). These polymers can also be cross-linked together to increase their viscosities and increase their proppant transport capabilities.
The plant based polysaccharides are linear or branched sugar polymers. Guar, for example, consists of a backbone of D-mannose residues bound to each other by a beta-1,4 linkage; molecules of in a ratio of 1:2 are also randomly attached to the backbone by an α-1,6 linkages. Cellulose, in contrast, is a polysaccharide consisting of a linear chain of several hundred to over ten thousand β(1→4) linked D-glucose units.
One of the issues that can arise in using polysaccharide and other thickeners that can be biologically degraded, is premature loss of viscosity. When oilfield produced water was used “as is” to prepare fracturing fluids, our studies have shown that the viscosity of the fluids usually quickly deteriorated in much the same manner as if a viscosity breaker had been prematurely activated in the fluid. Through a number of control experiments, we identified the likely cause of the fluid failure as the degradation of polysaccharide by bacteria and/or related enzymes (e.g., polysaccharidases) present in the produced water.
This suggests that before use, water should be treated with bactericides to avoid premature breakage. However, in our experiments, bactericides used at typical, anti-microbially effective concentrations were surprisingly found to have little or no effect on retaining fluid viscosity.
Thus, there is an unfulfilled need in the art for a cost-effective treatment of oilfield produced water so that the water can be used in the preparation of otherwise conventional highly viscous fracturing and other well treatment fluids and avoid premature loss of viscosity.